Seismic surveying



May 26, 1953 A. F. HAsBRo'oK l 2,640,186

SEISMIC SURVEYI'NG Filed June 13, 1949 walm. ML, 'f ATTORNEY UNITEDSTATES PATENT OFFICE SEISMIC SURVEYING Arthur F. Hasbroolt, San Antonio,Tex., assig'nor to Olive S. Petty, San Antonio, Tex.V

Application June 13, 1949, Serial No. 98,783

Claims.

This invention relates to methods and apparatus for use in seismicsurveying, and more especially to improvements in the practice ofrecording time-break signals, by means of which the instant ofdetonation of the explosive charge may be recorded and invariablydetermined with accuracy at a station remote from the blast.

In the field of seismic surveying, it is the customary practice togenerate seismic Waves in the earth by iii-ing a charge of highexplosive, such as dynamite or blasting gelatin, either adjacent to orbeneath the surface. The seismic waves so created are reflected upwardlyfrom various discontinuities beneath the surface and are received at aplurality of stations some distance from the source of the shock. Ateach receiving point, the seismic Waves are received by an instrument,sometimes called a seismozneter, and converted into electrical waveformenergy, which is amplified and recorded; a plurality of traces, eachrepresenting the seismic energy received at one of the severalseismometers, is commonly recorded on a single chart or seismogram. Inorder to interpret these traces, and especially to determine the variousdepths from which the seismic energy has been reiiected, it isessentialthat the exact time of firing the explosive charge be noted on the sameseismogram. v

In the practice most commonly followed, the instant of firing thecharge, or the time-break, is represented by a, single pulse on therecord, often transmitted from the shot-point to the recording stationby ratio, and is occasionally obscured by interfering signals or strayenergy. When this occurs, the seismic record is diflicult to interpretand may even be rendered Worthless. The obscuring of the time break is,of course, especially common and costly in exploration of the longdistance refraction type, in which radio transmission is essential.Various attempts, partly successful, have been made in the past toobviate these difficulties and to insure identification and exactlocation of the time-break signal, these including the use of mechanicalswitching units designed to give a, succession of signals, whereby theinstant of ring may be computed when the original time-break pulse isobscured. One of the difficulties encountered in the use of such devicesis the inevitable lack of timing accuracy in the generation of a seriesof signals by mechanical means.

It is, therefore, an object of the invention to` overcome these andother defects in the practice of recording time-break signals bygenerating electronic circuits a series of accurately spaced pulses,each bearing a definite relation in time to the original time-breakpulse, the original pulse and the separately generated series of pulsesbeing combined, preferably in opposite polarity to avoid confusion, andsuitably transmitted and recorded.

In the preferred form of the invention, the time-break pulse, which maybe derived in any convenient manner from the firing circuit or theblast, is applied to energize a damped Wave generator, by which a seriesof waves of decaying amplitude is produced. This series of waves is thenpassed through a pulse Shaper, and the resulting pulses are so combinedwith the original pulse that the latter may be distinguished by itspolarity.

As a further feature of the invention, transmission to the recordingstation of the timebreak signals so produced may be effected by applyingthe series of pulses to a carrier wave, generation or radiation of thecarrier being initiated by the original time-break pulse, while thefollowing series of pulses is applied as modulation on the carrier. Thusthe instant of arrival of the carrier wave at the remote recordingstation denotes the true time break, whereas the succeeding series ofpulses aords an accurate record from which the correct position of thetime break may be computed in the event the instant of arrival of thecarrier is obscured.

Further objects and features of the invention will be apparent from thefollowing description taken in connection with the accompanyingdrawings, in which Figure 1 represents diagrammatically a seismic systemsuitable for the practice of the invention, illustrating a preferredform of damped wave generator and pulse Shaper;

Figure 2 represents part of a seismogram on which appears a time-breaktrace such as may be produced by apparatus of the type shown in Figure1; and

Figure 3 is a, Wiring diagram of a preferred form of transmittingcircuit for use in the practice of the invention.

In order to promote an understanding of the invention, reference is madeto the specific embodiment thereof illustrated in the accompanyingdrawings and the same isi described in detail hereinafter. It willnevertheless be understood that such modifications and alterations ofthe invention are contemplated as would normally occurV to those skilledin the art to which the invention relates.

In Figure 1 is represented diagrammatically instance by combination inopposite polarity The time-break signals thus produced are sup-v pliedto a radio transmitter 20, by which they are radiated to a receiver 2I-,.11'ocated atthe remote recording station, and supplied to a c znveritionall seismic recorder 22.

Dealing more particularly with the Wave gen'- erating and pulse shapingcircuits,

source I which may be derivedff-romr the firing circuit as shown. in thepatents to Parr, Jr.. 2,331,623-, Petty-2,33r1,62'7, orA in any otherappropriate manner, is applied vthrough condenser I2 andacross resistorI4 to' the gridvof electron tube I3. The cathode of the tube isappropriately biased by resistors I7 and H rand the -tube outputis fedto a transformer. I5,.the primary wind-- ing of. which is shuntecl` .bycondenser H5,- the characteristics -of the -circuit being so chosen thatity functions as a dalnped Wave generator.l

Thus in thequiescent state, tubeA I3 -is blocked by the positive voltageimpressedu on the Acathode -throughresistor .network I1., I8.. theoriginal time-break pulse is applied-to the grid .of the tube, platecurrent surges.through the tuned circuit. comprisingtransformer I5 vand`I6,V the initial. voltage surgeat thecondenser plate .being negativebecause of. the large-drop in .potential across the reactive.tunedcircuit..

Oscillation of frequency, determinedv primarily by Ythe values of theprimary windingof Ytrans-l former I5 andof condenser Iii,` thenoccurs,`but 'I-hus is damped by the various `circuit losses. the output at theplate oftube I3 is approximately a sine Wave, rapidly decaying. to zeroamplitude.

preferably after a few cycles.

The damped VWave output is supplied from.

transformer I5 yto the grid. of electron tube. 1.9', which is normallyVheld at plate currentv cutoff by the bias voltage provided by resistorsmianclV 2'1. Thus tube I9 responds only. to the positive swings of theapplied damped Wave, and-...the output, consisting of predominantlynegative signals is applied to a differentiating circuit com-V prising..condenser 23 and resistor 2d, so .thatthe resulting signals appear inthe form of. a series of highly peaked negative pulses ofjdecreasingamplitude. These signals are .then applied to a grid of a multi-gridamplifier 25, theoriginal time-break pulse being applied to aseoond gridof the amplifier through condenser 2-8. In this manner the originaltime-break pulse and the succeeding series of generated. pulses arecombined in opposite polarity, as shown, and supplied to radiotransmitter 20' for radiation, re-y ception, vand recording.

In Figure-2 is shown a; part of a se'ismogram on which is represented atrace of the general type producedby the apparatus thusv far described,the original time-beak pulse being indicated at A and thesucceeding'serie's of' pulses ofi-opposite polarity and"decreasin'gamplitude ybeingkshown at B1, B2, and Bs. It"will"b`e" applate currentin.

When the positive .impulse-constituting it Will .be noted that theinitial time-break pulse from preciated that whenever the initial pulseA is obscured on the record, its exact location can be readilyidentified from the position of the pulses of the succeeding series, orsuch of these as have not been also obscured.

In its broadest aspects the invention contemplates the electronicgeneration, by other methods and means than those herein described, ofaf series: of '-pulses'ieachfbfearing avpred'eterrnined time relationto"the original time-break pulse for recording with the latter, whereby thecorrect position of said time-break pulse may be determined accuratelyeven When obscured.

In Figure 3 is represented a preferred method .o'f transmittingsignalenergy, generated as hereinbei'ore' described, for recording at aremote point. Thus the combined signal appearing in the output of tube25 may be applied to the grid oitube'30, functioning merely to amplifythe signal and reverse the polarity of the pulses. Thesignal-thus-amplied is thencsupplied through condenser- 3| to thegrid-.vofwth-yratron tube 33,- whichis normally biased off by means of aposi. tive voltage -developed throughupotentiometer 34,-

. Whichsupphes biasing voltage to the cathode of the tube. vIfhyiratron` 35iv lis connected in` series with tube 35, .thearrangement..beingfsuchthat plate .current flow. in `tube 35 `is'Vinitiated Vwhen thyratron.33 is red. .Control gridJli oftuvbe 35 issupplied. with Waveformi energy Vat theA desired carriervifrequency,so-thatthe-'carrier is transmitted through condenser 31anditransforrner` 3S to the antennaat the instant of arrival at the gridof thyratron 33` of the-positive pulse `which representsthe actual timebreak,...

It will beappreciated'. thatfthe remaining components of -the circuitvin..which..35fis included are those common to a modulated F. amplierstage, the modulatingsignal ybeing suppliedfrora tubeV 30' throughcondenser 44 .to .modulator-43 the series ofv negative pulses vwhicharel generated as, described with reference. to Figure .1. andreversedin.. polarity inn tube 3G."

By'lneans of the circuit just described, racha-- tion of the carrenis,initiatediby the. originalr time-break pulse, soasto indicate .abtheirecordin g. stationthe exact time vofii'ringthe charge. If the arrivalof. the carrier atthe .recordingsta-- tion is obscured because-ofinterference. the

exact instant of. detonati'oncan be computed from the position on therecord of' the pulses-oct the following series'.

I- Iaving thus described the invention, what is claimed as new andvdesired ,to vbe securedby,

Letters Patent' is:

1. Apparatus for use in .the recording of timebreak vsignals seismicsurveying includ-ing, in combination, an electronic circuitresponsive-to the time-break pulseto generate aseries ofpu-l-ses vhaving a .predetermined time nrelation` with saidrtime-break pulse toprovide asignal from which: the correct positionof thetime-breakpulse-can be determined regardless of. its obscurity; atrans--mitter radiating a carrier. signaly and Arnean'sY modulatingsaid carrierWithsaid-series of-pu-lses;

2. Apparatus for. use. in the recom-'dinger time-'- break signalsiii-.seismic surveyingincludingfincombination, anelectronic circuit`responsive to'. the tune-break pulse to generate a yseriesopulses fmitter radiating a carrier signal, means modulating said carrier withsaid series of pulses, and means combining in opposite polarity saidseparate pulses with said time-break pulse and applying the combinedsignal to said last named circuit and said modulating means.

3. Apparatus for use in the recording of timebreak signals in seismicsurveying including, in combination, a damped wave generator responsiveto the time-break pulse to generate a series of waves of decayingamplitude, a pulse shaper receiving and shaping said Waves into separatepulses to provide a signal from which the correct position of thetime-break pulse can be determined regardless of its obscurity, atransmitter radiating a carrier signal, and means modulating saidcarrier with said series of pulses.

4. Apparatus for use in the recording of timebreak signals in seismicsurveying including, in combination, an electronic circuit responsive tothe time-break pulse t0 generate a series of waves having apredetermined time relation to the timebreak pulse, a pulse shaperreceiving and shaping said waves into separate pulses, and meanscombining said separate pulses with sa-id timebreak pulse to provide asignal from which the correct position of the time-break pulse'can bedetermined regardless of its obscurity.

5. Apparatus for use in the recording of timebreak signals in seism-icsurveying including, in combination, a damped Wave generator responsiveto the time-break pulse to generate a series of Waves of decayingamplitude, a pulse shaper receiving and shaping said waves into separatepulses, and means combining in opposite polarity said separate pulseswith said time-break pulse to provide a signal from which the correctposition of the time-break pulse can be determined regardless of itsobscurity.

ARTHUR F. HASBROOK.

`References Cited in the le othis patent UNITED STATES PATENTS NumberName Date 2,084,845 Holmes June 22, 1937 2,250,834 Howton July 29, 19412,340,770 Reichert Feb. 1, 1944 2,348,525 Cravath May 9, 1944 2,435,903Ritzmann Feb. 10, 1948 2,493,379 Anderson Jan. 3, 1950 2,494,990 DeLario Jan. 17, 1950

